1. Field of the Invention
The present invention relates to an electron beam apparatus for focusing an electron beam onto a specimen with condenser lenses and an objective lens to image electrons transmitted through the specimen and, more particularly, to an electron beam apparatus which has an electron analyzer and acts to image only electrons having an energy coincident with the characteristic absorption energy of a certain element transmitted through the specimen.
2. Description of Related Art
Electron microscopes are roughly classified into two types: transmission electron microscope and scanning electron microscope. In a transmission electron microscope, an electron beam produced and accelerated from an electron gun is focused onto a specimen by condenser lenses and an objective lens. Electrons transmitted through the specimen or scattered electrons are imaged onto a fluorescent screen, photographic film, or TV camera.
Sometimes, such a transmission electron microscope is fitted with an energy filter. In this instrument fitted with the energy filter, only electrons transmitted through a specimen and having characteristic absorption energy coincident with that of a certain element are imaged. Thus, information about the specimen, such as elemental distribution, can be obtained. The phenomenon where the energies of electrons are absorbed by a specimen is known as energy loss. Analysis of the energy loss is known as energy loss spectroscopy.
This energy filter is often mounted behind the image. The image is detected by an image tube or the like by making use of electrons transmitted through an energy-selecting slit or aperture baffle. A certain energy loss image can be derived by appropriately setting the width and position of the slit.
Although an energy loss image of a certain element in the specimen can be obtained in this way, the background level of the energy loss spectrum is high in practice. It is essential to subtract the background to extract information about the certain element from such a spectrum. Some methods are used in practice to subtract the background in this manner. These methods are described below.
First, the two-window method is described by referring to FIG. 1, which is an illustration of energy loss spectrum. In this graph, the energy loss value is plotted on the horizontal axis and the electron intensity corresponding to each energy loss value on the vertical axis. In the case of this two-window method, an image (A) of energy loss value of interest is obtained. In addition, an image (B) of a lower energy loss value is obtained. Signal processing given by A−B or A/B is performed and thus information about a desired certain element is obtained. The latter calculation is performed to acquire an intensity ratio rather than background subtraction. Therefore, the ratio may be referred to as the jump ratio.
Now, the three-window method is described by referring to FIG. 2. In this method, an image (A) of an energy loss value of interest is obtained. In addition, two images (B) and (C) of lower energy loss values are taken. The background of the image (A) is estimated from the two image signals (B) and (C). The value of the background is indicated by (D). Signal processing given by A−D is performed.
In the above-described estimation of the background, the relation between the intensity of energy-loss spectrum and energy-loss value is derived. This depends on the actual electron energy and on the state of the specimen, i.e., contained elements and thickness of the specimen. A calculation must be performed for each individual appropriate model of the interaction between material and electron.
Some interaction models have been proposed. Any one of them is used according to the element or energy to be imaged. Since such an interaction model is not directly related to the present invention, its detailed description is omitted. In any case, however, the loss energy value is switched, and plural filtered images are acquired. Computational processing is performed between the gained image signals.
When images of differing losses are obtained, electrons for generating the images are introduced into an analyzer, which energy-disperses the electrons. Only those of the energy-dispersed electrons which have a certain energy value are passed through an output slit located after the analyzer. An image is obtained only from electrons having the certain energy value by an image pickup device, such as a CCD camera. Often, a sector magnetic field is used as this analyzer. The energy of the electrons passed through the output slit is varied by varying the strength of the magnetic field.
An example of an electron microscope fitted with this analyzer is shown in FIG. 3, where electrons accelerated from an electron gun 1 with an accelerating voltage E are illuminated on a specimen 3 as an electron beam by an illumination lens system 2. This illumination lens system 2 includes a combination of condenser lenses and a magnetic pre-field of the objective lens.
Electrons transmitted through the specimen 3 or scattered electrons are imaged onto the incident aperture baffle 6 of the analyzer 5 by an imaging lens system 4. The energies of some of the electrons are absorbed on passing through the specimen, while the energies of other electrons are absorbed according to the elements constituting the specimen. The electrons passed through an opening of the incident aperture baffle 6 enter the analyzer 5.
A magnetic field is set up within the analyzer 5. The electrons incident on the analyzer 5 are deflected by the magnetic field. The angle through which the electrons are deflected differs depending on the energy. That is, the electrons are energy-dispersed by the analyzer 5. A slit baffle 7 is mounted on the exit side of the analyzer 5. The electrons passed through an opening of the slit baffle 7 are only electrons having energy E corresponding to the strength of the magnetic field in the analyzer 5.
Electrons which have been deflected greatly by the analyzer 5 and have energies (E−δE) smaller than energy E are blocked out by the slit baffle 7. The electrons passed through the slit baffle 7 and having the certain energy E are imaged onto the sensitive surface of the image recording device 9, such as a CCD camera, by an imaging lens system 8. As a result, electrons having a certain energy are detected as an image by the image recording device 9. In this case, if the strength of the magnetic field forming the analyzer 5 is swept, the energy of the electron passed through the opening of the slit baffle 7 is varied by varying the strength of the magnetic field.
The analyzer 5 described above forms a sector-shaped magnetic field, and the strength of the magnetic field is varied. The following configuration is also possible. The strength of the magnetic field is maintained constant. An electrically conductive tube is mounted in the electron passage within the analyzer. A constant potential is applied to the tube from a power supply 10 shown in FIG. 4 to vary the electron energy temporarily. The energy of the electron passed through the opening of the slit baffle 7 is swept. In the example of FIG. 4, the potential inside the analyzer 5 is increased, and electrons having lower energy (E−δE) are passed through the slit baffle 7. Electrons having higher energy E are blocked out by the slit baffle 7.
FIG. 5 shows another example in which the energy of the electron passed through the opening of the slit baffle 7 is varied. In the configuration of this FIG. 5, the slit baffle 7 is made movable relative to the front and rear stages of electron optics. If the slit baffle 7 is moved in the direction of the arrow in the figure, electrons having different energies can be selectively passed through the opening of the slit baffle 7. In the example of FIG. 5, the opening of the slit baffle 7 is moved into the position where the electrons of the lower energy (E−δE) are imaged. On the other hand, the electrons having the energy E and imaged onto the optical axis are blocked out by the slit baffle 7.
FIG. 6 shows an example in which the energy is selected without varying the conditions of the analyzer 5 and without mechanically moving the slit baffle 7. In the configuration of this FIG. 6, a deflection coil 11 is disposed between the analyzer 5 and slit baffle 7. Electrons exiting from the analyzer 5 and dispersed are deflected by the deflection coil 11. Thus, electrons having different energies can be passed through the opening of the slit baffle 7.
FIG. 7 shows an example in which the energy is selected without varying the conditions of the analyzer 5, without mechanically moving the slit baffle 7, and without using a deflection coil. In the configuration of this FIG. 7, the accelerating voltage of the electron gun 1 is varied to change the energy of the electrons illuminating on the specimen. For example, the voltage with which the electrons are accelerated in the electron gun 1 is varied from E to E′ (E′=E+δE) (increased in this case).
Consequently, the spectrum on the slit baffle 7 shifts. The energy loss value of the electrons passed through the slit coincides with the increment δE in the illuminating energy. That is, electrons passed through the opening of the slit baffle 7 have energy E. Electrons having the energy E and passed through the slit baffle 7 up to now come to have energy of E+δE. In consequence, the electrons are blocked off by the slit baffle 7. On the other hand, electrons having energy of E′−δE come to haveE′−δE=(E+δE)−δE=EAs a result, the electrons are bent by the analyzer 5 and pass through the opening of the slit baffle 7 on the optical axis. In this way, electrons can also be passed through an electronic slit of desired energy loss value by varying the accelerating voltage of the electron gun 1.
Electron microscopes fitted with the aforementioned energy filter are disclosed in Japanese Patent Laid-Open No. 2000-268766 and Japanese Patent Laid-Open No. H11-86771. Where an image is formed by selecting electrons of a certain energy, a tube is mounted in a sector-shaped magnetic field in the beam path. A voltage is applied to the tube to vary the energy of the electrons. Moreover, a system in which a filter, such as an Ω-filter, α-filter, or γ-filter, is positioned in the electron optical system is used.
As mentioned previously, four methods are conceivable to switch the loss energy. In practice, these methods are in operation. In the first method, one condition of the analyzer 5 (e.g., the strength of the sector-shaped magnetic field) is varied as shown in FIGS. 3 and 4 or a certain voltage is applied to the beam path in the analyzer and the energy of the electron is varied temporarily, thus moving the spectrum. In the second method, the exit slit baffle 7 mounted in the rear stage of the analyzer 5 shown in FIG. 5 is moved mechanically.
In the third method, the deflection coil 11 is mounted between the analyzer 5 and slit baffle 7 as shown in FIG. 6. In the fourth method, the energy of the electron beam illuminated on the specimen 3 is varied by varying the accelerating voltage of the electron gun 1 as shown in FIG. 7.
Of the four methods described above, the first and fourth methods have been performed widely. In the second method, the slit baffle 7 is moved mechanically and therefore, if the accuracy at which the mechanical movement is made is enhanced to a quite high level, the accuracy is unsatisfactory compared with the energy resolution. Furthermore, the reproducibility of image presents a problem. In addition, extra cost is spent for the moving mechanism.
In the third method, the position of the opening of the slit fails to agree with the optical axis of the imaging lens system mounted behind the slit baffle 7 and so aberration and axial misalignment occur. In this way, the second and third methods have great problems. Consequently, the first and fourth methods are used but they still have both advantages and disadvantages.
For example, in the first method, the spectrum can be moved with high reproducibility by sweeping the magnetic field in the analyzer or by maintaining the magnetic field constant and applying a potential to the tube in the beam passage within the analyzer 5. Also, axial misalignment of the electrons passed through the slit baffle 7 with respect to the imaging lens system 8 after the analyzer 5 is not produced. Furthermore, no axial misalignment occurs in the illumination lens system 2 or imaging lens system 4 before the analyzer 5 because the set conditions are not varied at all.
However, in both the imaging lens system 8 after the analyzer 5 and the imaging lens system 4 before the analyzer 5, conditions (e.g., focusing) are accurately set for electrons without energy loss (zero-loss electrons) before a potential is applied to the tube in the analyzer 5. Accordingly, where the tube potential is varied, if the energy of the electrons imaged is varied by applying the tube potential, the set conditions are no longer satisfied for the electrons unless all other conditions for the lenses and deflection system are varied in relation with the tube potential. That is, defocusing occurs.
In the fourth method, the accelerating voltage of the electron beam illuminated on the specimen 3 is varied. The conditions for the illumination lens system 2 in front of the specimen 3 are no longer satisfied, producing axial misalignment. However, after transmission through the specimen, desired energy-loss electrons have an actual energy coincident with the lens conditions and so the image is not defocused. Accordingly, the fourth method is generally adopted in an energy filter that selects electrons of a desired energy with the energy-selecting slit baffle 7 and brings the electrons to an image.
As mentioned previously, the problem with the fourth method is that the conditions of the illuminating lens system 2 deviate. This may shift the region on the specimen 3 illuminated with the electron beam or the brightness of the illuminating electron beam may vary, degrading the accuracy of signal processing. Therefore, a method of providing feedback control has been proposed. In particular, the conditions of the illumination optical system including the illuminating lens system 2 and deflection coil 40 for axial alignment are varied according to variation of the accelerating voltage such that the region on the specimen illuminated with the beam and the brightness of the illuminating beam remain unchanged if the accelerating voltage of the electron beam is varied or increased.
As described above, where the accelerating voltage of the electron beam is varied, it is necessary to vary the operating conditions of the illumination optical system 2, because the strengths of the lenses and the strength of the deflection coil are in proportion to the square root of the relativistic energy of each lens. More specifically, let E be the energy prior to increase of the accelerating voltage. Let E* be the relativistic energy. Let E′ (=E+δE) be the energy of the electron beam after the accelerating voltage is increased. Let E′* be the relativistic value of this energy E′. There is the following relation among the energy not yet increased, the current I flowing into the lenses and deflection coil, and the current I′ flowing into them after the increase:
            I      ′        I    =                    E                  ′          *                            E        *            For these reasons, where the accelerating voltage of the electron beam is varied, the operating conditions of the lenses and deflection coil of the illumination optical system are controlled by feedback to prevent positional deviation of the electron beam on the specimen 3 and brightness variations.
In the normal transmission electron microscope, the specimen is placed within the magnetic field of the objective lens 20. The magnetic field before the specimen acts as an illumination lens. The magnetic field after the specimen acts as an imaging lens. This means that correct operation cannot be expected unless feedback to the illumination optical system located ahead of the specimen is also applied to the objective lens 20. It is impossible, however, in practice to control the illuminating action of the objective lens 20 and the imaging action separately. In spite of this, if the strength of the magnetic field of the objective lens 20 ahead of the specimen 3 is controlled by feedback according to variation of the accelerating voltage, the imaging action of the magnetic field of the objective lens 20 behind the specimen 3 is adversely affected. This defocuses the image. As a result, the purpose cannot be achieved with the feedback to the illuminating lens system.